We have developed techniques to label, specifically and radioactively, carbohydrate portions of cell membrane glycoproteins and glycolipids. By applying such methods followed by polyacrylamide gel electrophoresis and autoradiography, we have previously shown that purified populations of normal human blood leukocytes have characteristic and distinguishable surface glycoprotein patterns that clearly correlate with the stage of cellular maturation and activation and have shown that various cultured benign and malignant leukocyte cell lines can be characterized by their surface glycoprotein profiles. We have shown that the human leukemia cell line K562 is erythroid, can be induced to erythrocyte differentiation in vitro, and is useful for the study of erythroid patterns. We have studied: (1)\O-glycosylation of glycophorin A in K562 cells. The results clearly show that the initiation of O-glycosylation is an early (pre-Golgi) biosynthetic event that is not inhibited by monensin; (2)\the major surface-labeled sialoglycoprotein of human leukocytes has been isolated and a heteroantiserum prepared. The apparent molecular weight of the protein changes during cellular differentiation, and this seems to be due to varying amounts of carbohydrate; (3)\we have shown that the extent of glycophorin A glycosylation depends on the level of differentiation of the erythroid cells of origin; (4)\the Rho(D) antigen from red cellshas been isolated, partially characterized; and although a surface protein, it may lack carbohydrate, and it is anchored to the membrane skeleton; and (5) several granulocyte/monocyte differentiation antigens have been partially characterized at the molecular level. (A)